In the present research, the effects of surface rock fragments and soil clay content on surface runoff and soil loss was investigated under the laboratory conditions. The aim of the test was to increase the general understanding of how soil clay content and surface rock fragments affect the soil erosion process. A rainfall simulator was added to an erosion plot and these apparatuses were used to investigate the effects of varying soil clay content (SCC) and soil rock fragments (SRF) on soil erosion by measuring runoff volume and sediment yield at regular time intervals during the simulation. The results indicated that the main effects of soil clay content and surface rock fragments were all significant at the 0.95 level (p<0.05) for the runoff and sediment yield. Also, the most significant factor was the quantity of soil clay content in comparison with the surface rock fragments. The interaction effect between surface rock fragments and soil clay content was not significant for the runoff volume, but in case of sediment yield it had a great influence. The repeated measures analysis of variance for time intervals revealed that the main effects of sampling time and its interactions with soil clay content and surface rock fragments were all significant (p<0.05) as well, although the effects of time intervals reduced gradually while the rainfall simulation proceeded. The results indicated that the main effects and interactions must be accounted for any simulation study of soil erosion and modeling, and the multiple effects should be studied in research rather than the isolated effects of single variables.

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